Search

Recent studies have demonstrated the potential of antiferromagnets as the active component in spintronic devices. This is in contrast to their current passive role as pinning layers in hard disk read heads and magnetic memories. Here we report the epitaxial growth of a new high-temperature antiferromagnetic material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical order and compatibility with existing semiconductor technologies. We demonstrate its growth on the III-V semiconductors GaAs and GaP, and show that the structure is also lattice matched to Si. Neutron diffraction shows collinear antiferromagnetic order with a high Ne\'el temperature. Combined with our demonstration of room-temperature exchange coupling in a CuMnAs/Fe bilayer, we conclude that tetragonal CuMnAs films are suitable candidate materials for antiferromagnetic spintronics., Comment: 16 pages, 5 figures, Published in Nature Communications (2013)

We investigate the relationship between the Curie temperature TC and the carrier density p in the ferromagnetic semiconductor (Ga,Mn)As. Carrier densities are extracted from analysis of the Hall resistance at low temperatures and high magnetic fields. Results are found to be consistent with ion channeling measurements when performed on the same samples. We find that both TC and the electrical conductivity increase monotonically with increasing p, and take their largest values when p is comparable to the concentration of substitutional Mn acceptors. This is inconsistent with models in which the Fermi level is located within a narrow isolated impurity band., Comment: 10 pages, 4 figures

Electrical current manipulation of magnetization switching through spin-orbital coupling in ferromagnetic semiconductor (Ga,Mn)As Hall bar devices has been investigated. The efficiency of the current-controlled magnetization switching is found to be sensitive to the orientation of the current with respect to the crystalline axes. The dependence of the spin-orbit effective magnetic field on the direction and magnitude of the current is determined from the shifts in the magnetization switching angle. We find that the strain induced effective magnetic field is about three times as large as the Rashba induced magnetic field in our GaMnAs devices., Comment: 11 pages,3 figures, submitted to Appl. Phys. Lett

Comment on the recent Nature Materials article by M. Dobrowolska et al., arXiv:1203.1852. We present experimental data showing that the Curie temperature and conductivity of high quality (Ga,Mn)As samples are maximized at low compensation, and thus the magnetic order in (Ga,Mn)As is not consistent with the isolated impurity band scenario., Comment: 6 pages, 2 figures

We analyze microscopically the valence and impurity band models of ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with conventional parameterization and the full potential LDA+U calculations give a very similar picture of states near the Fermi energy which reside in an exchange-split sp-d hybridized valence band with dominant orbital character of the host semiconductor; this microscopic spectral character is consistent with the physical premise of the k.p kinetic-exchange model. On the other hand, the various models with a band structure comprising an impurity band detached from the valence band assume mutually incompatible microscopic spectral character. By adapting the tight-binding Anderson calculations individually to each of the impurity band pictures in the single Mn impurity limit and then by exploring the entire doping range we find that a detached impurity band does not persist in any of these models in ferromagnetic (Ga,Mn)As., Comment: 29 pages, 25 figures

After decades of research, the low Curie temperature of ferromagnetic semiconductors remains the key problem in the development of magnetic semiconductor spintronic technologies. Removing this roadblock might require a change of the field's basic materials paradigm by looking beyond ferromagnets. Recent studies of relativistic magnetic and magnetotransport anisotropy effects, which in principle are equally well present in materials with ferromagnetically and antiferromagnetically ordered spins, have inspired our search for antiferromagnetic semiconductors suitable for high-temperature spintronics. Since these are not found among the magnetic counterparts of common III-V or II-VI semi- conductors, we turn the attention in this paper to high N \'eel temperature I-II-V magnetic compounds whose electronic structure has not been previously identified. Our combined experimental and theoretical work on LiMnAs provides basic prerequisite for the systematic research of this class of materials by demonstrating the feasibility to grow single crystals of group-I alkali metal compounds by molecular beam epitaxy, by demonstrating the semiconducting band structure of the I-Mn-V's, and by analyzing their spin-orbit coupling characteristics favorable for spintronics., Comment: 11 pages, 5 figures

We have investigated the effects of post growth low temperature annealing on the magnetic, electrical and structural properties of (Ga_0.94,Mn_0.06)(As_0.9,P_0.1) layers grown by molecular beam epitaxy. By controlling the annealing time we are able to tune the magnetic anisotropy between an easy axis in the plane for the as-grown samples, to an easy axis perpendicular to the plane for fully annealed samples. The increase of the carrier density, as a result of annealing, is found to be the primary reason for the change in magnetic anisotropy, in qualitative agreement with theoretical predictions., Comment: 13 pages, 3 figures, submitted to Applied Physics Letters

The quantum well solar cell (QWSC) has been proposed as a route to higher efficiency than that attainable by homojunction devices. Previous studies have established that carriers escape the quantum wells with high efficiency in forward bias and contribute to the photocurrent. Progress in resolving the efficiency limits of these cells has been dogged by the lack of a theoretical model reproducing both the enhanced carrier gen- eration and enhanced recombination due to the quantum wells. Here we present a model which calculates the incremental generation and recombination due to the QWs and is verified by modelling the experimental light and dark current-voltage characteristics of a range of III-V quantum well structures. We find that predicted dark currents are significantly greater than experiment if we use lifetimes derived from homostructure devices. Successful simulation of light and dark currents can be obtained only by introducing a parameter which represents a reduction in the quasi-Fermi level separation., Comment: Preprint submitted to the 28th IEEE Photovoltaic Specialists Conference, Anchorage, Alaska, USA, Sept. 2000, pp. 1304-1307

Using x-ray magnetic circular dichroism (XMCD), we determine the element-specific character and polarization of unoccupied states near the Fermi level in (Ga,Mn)As and (In,Ga,Mn)As thin films. The XMCD at the As K absorption edge consists of a single peak located on the low-energy side of the edge, which increases with the concentration of ferromagnetic Mn moments. The XMCD at the Mn K edge is more detailed and is strongly concentration-dependent, which is interpreted as a signature of hole localization for low Mn doping. The results indicate a markedly different character of the polarized holes in low-doped insulating and high-doped metallic films, with a transfer of the hole orbital magnetic moment from Mn to As sites on crossing the metal-insulator transition., Comment: 5 figures, to be published in Physical Review B

Current-driven magnetic domain wall motion is demonstrated in the quaternary ferromagnetic semiconductor (Ga,Mn)(As,P) at temperatures well below the ferromagnetic transition temperature, with critical currents of the order 10^5Acm^-2. This is enabled by a much weaker domain wall pinning compared to (Ga,Mn)As layers grown on a strain-relaxed buffer layer. The critical current is shown to be comparable with theoretical predictions. The wide temperature range over which domain wall motion can be achieved indicates that this is a promising system for developing an improved understanding of spin-transfer torque in systems with strong spin-orbit interaction., Comment: 12 pages, 3 figures

We have investigated the domain wall resistance for two types of domain walls in a (Ga,Mn)As Hall bar with perpendicular magnetization. A sizeable positive intrinsic DWR is inferred for domain walls that are pinned at an etching step, which is quite consistent with earlier observations. However, much lower intrinsic domain wall resistance is obtained when domain walls are formed by pinning lines in unetched material. This indicates that the spin transport across a domain wall is strongly influenced by the nature of the pinning., Comment: 9 pages, 3 figures

We demonstrate an exchange bias in (Ga,Mn)As induced by antiferromagnetic coupling to a thin overlayer of Fe. Bias fields of up to 240 Oe are observed. Using element-specific x-ray magnetic circular dichroism measurements, we distinguish a strongly exchange coupled (Ga,Mn)As interface layer in addition to the biassed bulk of the (Ga,Mn)As film. The interface layer remains polarized at room temperature., Comment: 3 figures, submitted to Physical Review B

We demonstrate a simple, low cost, magneto-transport method for rapidly characterizing the magnetic anisotropy and anisotropic magneto-resistance (AMR) of ferromagnetic devices with uniaxial magnetic anisotropy. This transport technique is the analogue of magnetic susceptibility measurements of bulk material but is applicable to very small samples with low total moment. The technique is used to characterize devices fabricated from the dilute magnetic semiconductor (Ga,Mn)As. The technique allows us to probe the behavior of the parameters close to the Curie temperature, in the limit of the applied magnetic field tending to zero. This avoids the complications arising from the presence of paramagnetism.

We have studied the magnetic reversal of L-shaped nanostructures fabricated from (Ga,Mn)As. The strain relaxation due to the lithographic patterning results in each arm having a uniaxial magnetic anisotropy. Our analysis confirms that the magnetic reversal takes place via a combination of coherent rotation and domain wall propagation with the domain wall positioned at the corner of the device at intermediate stages of the magnetic hysteresis loops. The domain wall energy can be extracted from our analysis. Such devices have found implementation in studies of current induced domain wall motion and have the potential for application as non-volatile memory elements.

We study the effects of growth temperature, Ga:As ratio and post-growth annealing procedure on the Curie temperature, Tc, of (Ga,Mn)As layers grown by molecular beam epitaxy. We achieve the highest Tc values for growth temperatures very close to the 2D-3D phase boundary. The increase in Tc, due to the removal of interstitial Mn by post growth annealing, is counteracted by a second process which reduces Tc and which is more effective at higher annealing temperatures. Our results show that it is necessary to optimize the growth parameters and post growth annealing procedure to obtain the highest Tc.

We present an experimental investigation of the magnetic, electrical and structural properties of Ga0.94Mn0.06As1-yPy layers grown by molecular beam epitaxy on GaAs substrates for y less than or equal to 0.3. X-ray diffraction measurements reveal that the layers are under tensile strain which gives rise to a magnetic easy axis perpendicular to the plane of the layers. The strength of the magnetic anisotropy and the coercive field increase as the phosphorous concentration is increased. The resistivity of all samples shows metallic behaviour with the resistivity increasing as y increases. These materials will be useful for studies of micromagnetic phenomena requiring metallic ferromagnetic material with perpendicular magnetic anisotropy.

We have studied current-driven domain wall motion in modified Ga_0.95Mn_0.05As Hall bar structures with perpendicular anisotropy by using spatially resolved Polar Magneto-Optical Kerr Effect Microscopy and micromagnetic simulation. Regardless of the initial magnetic configuration, the domain wall propagates in the opposite direction to the current with critical current of 1~2x10^5A/cm^2. Considering the spin transfer torque term as well as various effective magnetic field terms, the micromagnetic simulation results are consistent with the experimental results. Our simulated and experimental results suggest that the spin-torque rather than Oersted field is the reason for current driven domain wall motion in this material., Comment: 10 pages, 3 figures, to be published in J. Magn. Magn. Mater

We investigate the anisotropy of magnetic reversal and current-driven domain wall motion in annealed Ga_0.95Mn_0.05As thin films and Hall bar devices with perpendicular magnetic anisotropy. Hall bars with current direction along the [110] and [1-10] crystallographic axes are studied. The [110] device shows larger coercive field than the [1-10] device. Strong anisotropy is observed during magnetic reversal between [110] and [1-10] directions. A power law dependence is found for both devices between the critical current (JC) and the magnetization (M), with J_C is proportional to M^2.6. The domain wall motion is strongly influenced by the presence of local pinning centres., Comment: 12 pages, 12 figures, submitted to New Journal of Physics

We report large anisotropic magnetoresistance (AMR) behaviours in single lateral (Ga,Mn)As nanoconstriction of up to 1300%, along with large multistable telegraphic switching. The nanoconstriction devices are fabricated using high-resolution electron beam lithography of a 5 nm thick (Ga,Mn)As epilayer. The unusual behaviour exhibited by these devices is discussed in the context of existing theories for enhanced AMR ferromagnetic semiconductor nanoscale devices, particularly with regard to the dependence on the magnetotransport of the bulk material. We conclude that our results are most consistent with the Coulomb blockade AMR mechanism., Comment: 10 pages, 5 figures

It has been demonstrated that magnetocrystalline anisotropies in (Ga,Mn)As are sensitive to lattice strains as small as 10^-4 and that strain can be controlled by lattice parameter engineering during growth, through post growth lithography, and electrically by bonding the (Ga,Mn)As sample to a piezoelectric transducer. In this work we show that analogous effects are observed in crystalline components of the anisotropic magnetoresistance (AMR). Lithographically or electrically induced strain variations can produce crystalline AMR components which are larger than the crystalline AMR and a significant fraction of the total AMR of the unprocessed (Ga,Mn)As material. In these experiments we also observe new higher order terms in the phenomenological AMR expressions and find that strain variation effects can play important role in the micromagnetic and magnetotransport characteristics of (Ga,Mn)As lateral nanoconstrictions., Comment: 11 pages, 4 figures, references fixed

There is currently much interest in materials and structures that provide coupled ferroelectric and ferromagnetic responses, with a long-term goal of developing new memories and spintronic logic elements. Within the field there is a focus on composites coupled by magnetostrictive and piezoelectric strain transmitted across ferromagnetic-ferroelectric interfaces, but substrate clamping limits the response in the supported multilayer configuration favoured for devices. This constraint is avoided in a ferroelectric-ferromagnetic bilayer in which the magnetic response is modulated by the electric field of the poled ferroelectric. Here, we report the realization of such a device using a diluted magnetic semiconductor (DMS) channel and a polymer ferroelectric gate. Polarization reversal of the gate by a single voltage pulse results in a persistent modulation of the Curie temperature as large as 5%. The device demonstrates direct and quantitatively understood electric-fieldmediated coupling in a multiferroic bilayer and may provide new routes to nanostructured DMS materials and devices via ferroelectric domain nanopatterning. The successful implementation of a polymer-ferroelectric gate fieldeffect transistor (FeFET) with a DMS channel adds a new functionality to semiconductor spintronics and may be of importance for future low-voltage spintronics devices and memory structures., Comment: 19 pages, 5 figures

We demonstrate sensitivity of the Mn 3d valence states to strain in the ferromagnetic semiconductors (Ga,Mn)As and (Al,Ga,Mn)As, using x-ray magnetic circular dichroism (XMCD). The spectral shape of the Mn $L_{2,3}$ XMCD is dependent on the orientation of the magnetization, and features with cubic and uniaxial dependence are distinguished. Reversing the strain reverses the sign of the uniaxial anisotropy of the Mn $L_3$ pre-peak which is ascribed to transitions from the Mn 2p core level to p-d hybridized valence band hole states. With increasing carrier localization, the $L_3$ pre-peak intensity increases, indicating an increasing 3d character of the hybridized holes., Comment: 4 pages plus 2 figures, accepted for publication in Physical Review B

We demonstrate dynamic voltage control of the magnetic anisotropy of a (Ga,Mn)As device bonded to a piezoelectric transducer. The application of a uniaxial strain leads to a large reorientation of the magnetic easy axis which is detected by measuring longitudinal and transverse anisotropic magnetoresistance coefficients. Calculations based on the mean-field kinetic-exchange model of (Ga,Mn)As provide microscopic understanding of the measured effect. Electrically induced magnetization switching and detection of unconventional crystalline components of the anisotropic magnetoresistance are presented, illustrating the generic utility of the piezo voltage control to provide new device functionalities and in the research of micromagnetic and magnetotransport phenomena in diluted magnetic semiconductors., Comment: Submitted to Physical Review Letters. Updates version 1 to include a more detailed discussion of the effect of strain on the anisotropic magnetoresistance

The element-specific technique of x-ray magnetic circular dichroism (XMCD) is used to directly determine the magnitude and character of the valence band orbital magnetic moments in (III,Mn)As ferromagnetic semiconductors. A distinct dichroism is observed at the As K absorption edge, yielding an As 4p orbital magnetic moment of around -0.1 Bohr magnetons per valence band hole. This is strongly influenced by strain, indicating its crucial influence on the magnetic anisotropy. The dichroism at the Ga K edge is much weaker. The K edge XMCD signals for Mn and As both have positive sign, which indicates the important contribution of Mn 4p states to the Mn K edge spectra., Comment: 5 pages, 2 figures

We present details of our experimental and theoretical study of the components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. We develop experimental methods to yield directly the non-crystalline and crystalline AMR components which are then independently analyzed. These methods are used to explore the unusual phenomenology of the AMR in ultra thin (5nm) (Ga,Mn)As layers and to demonstrate how the components of the AMR can be engineered through lithography induced local lattice relaxations. We expand on our previous [Phys. Rev. Lett. \textbf{99}, 147207 (2007)] theoretical analysis and numerical calculations to present a simplified analytical model for the origin of the non-crystalline AMR. We find that the sign of the non-crystalline AMR is determined by the form of spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential., Comment: Submitted to JMMM for conference proceedings of WUN-SPIN 2007 (York, UK)

The large saturation magnetization in conventional dense moment ferromagnets offers flexible means of manipulating the ordered state through demagnetizing shape anisotropy fields but these dipolar fields, in turn, limit the integrability of magnetic elements in information storage devices. We show that in a (Ga,Mn)As dilute moment ferromagnet, with comparatively weaker magnetic dipole interactions, locally tunable magnetocrystalline anisotropy can take the role of the internal field which determines the magnetic configuration. Experiments and theoretical modeling are presented for lithographically patterned microchannels and the phenomenon is attributed to lattice relaxations across the channels. The utility of locally controlled magnetic anisotropies is demonstrated in current induced switching experiments. We report structure sensitive, current induced in-plane magnetization switchings well below the Curie temperature at critical current densities 10^5 Acm^-2. The observed phenomenology shows signatures of a contribution from domain-wall spin-transfer-torque effects., Comment: 9 pages, 9 figures, Phys. Rev. B in press

We discuss the character of states near the Fermi level in Mn doped GaAs, as revealed by a survey of dc transport and optical studies over a wide range of Mn concentrations. A thermally activated valence band contribution to dc transport, a mid-infrared peak at energy hbar omega approx 200 meV in the ac- conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low doped (<<1% Mn) insulating GaAs:Mn materials. Consistent with the implications of this picture, both the impurity band ionization energy inferred from the dc transport and the position of the mid-infrared peak move to lower energies and the peak broadens with increasing Mn concentration. In metallic materials with > 2% doping, no traces of Mn-related activated contribution can be identified in dc-transport, suggesting that the impurity band has merged with the valence band. No discrepancies with this perception are found when analyzing optical measurements in the high-doped GaAs:Mn. A higher energy (hbar omega approx 250 meV) mid-infrared feature which appears in the metallic samples is associated with inter-valence band transitions. Its red-shift with increased doping can be interpreted as a consequence of increased screening which narrows the localized-state valence-band tails and weakens higher energy transition amplitudes. Our examination of the dc and ac transport characteristics of GaAs:Mn is accompanied by comparisons with its shallow acceptor counterparts, confirming the disordered valence band picture of high-doped metallic GaAs:Mn material., Comment: 10 pages, 12 figures

We have performed spatially resolved Polar Magneto-Optical Kerr Effect Microscopy measurements on as-grown and annealed Ga0.95Mn0.05As thin films with tensile strain. We find that the films exhibit very strong perpendicular magnetic anisotropy which is increased upon annealing. During magnetic reversal, the domain walls propagate along the direction of surface ripples for the as-grown sample at low temperatures and along the [110] direction for the annealed sample. This indicates that the magnetic domain pattern during reversal is determined by a combination of magnetocrystalline anisotropy and a distribution of pinning sites along the surface ripples that can be altered by annealing. These mechanisms could lead to an effective method of controlling domain wall propagation., Comment: 8 pages, 3 figures. to appear in Journal of Applied Physics

Our experimental and theoretical study of the non-crystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As is aimed at exploring the basic physical aspects of this relativistic transport effect. The non-crystalline AMR reflects anisotropic lifetimes of the holes due to polarized Mn impurities while the crystalline AMR is associated with valence band warping. We find that the sign of the non-crystalline AMR is determined by the form of spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential. We develop experimental methods directly yielding the non-crystalline and crystalline AMR components which are then independently analyzed. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. We discuss generic implications of our experimental and theoretical findings including predictions for non-crystalline AMR sign reversals in dilute moment systems., Comment: 4 pages, 3 figures. Phys. Rev. Lett. in press

We have measured the Anisotropic Magnetoresistance (AMR) of ultra thin (5nm) Ga(0.95)Mn(0.05)As films. We find that the sign of the AMR can be positive or negative, which may depend on the direction of the current with respect to the crystal. At low temperatures, transport measurements and SQUID magnetometry suggest that the magnetisation has a component pointing out of the plane of the film., Comment: Accepted for publication in Physica Status Solidi C

Single-electronics and spintronics are among the most intensively investigated potential complements or alternatives to CMOS electronics. Single-electronics, which is based on the discrete charge of the electron, is the ultimate in miniaturization and electro-sensitivity. Spintronics, which is based on manipulating electron spins,delivers high magneto-sensitivity and non-volatile memory effects. So far, major developments in the two fields have followed independent paths with only a few experimental studies of hybrid single-electronic/spintronic devices. Intriguing new effects have been discovered in such devices but these have not, until now, offered the possibility of useful new functionalities. Here we demonstrate a device which shows a new physical effect, Coulomb blockade anisotropic magnetoresistance, and which offers a route to non-volatile, low-field, and highly electro- and magneto-sensitive operation. Since this new phenomenon reflects the magnetization orientation dependence of the classical single-electron charging energy it does not impose constraints on the operational temperature associated with more subtle quantum effects, such as resonant or spin-coherent tunneling.

I. Introduction II. Theoretical background (Landauer-Buttiker formalism of thermo-electricity, Quantum point contacts as ideal electron waveguides, Saddle-shaped potential) III. Experiments (Thermopower, Thermal conductance, Peltier effect) IV. Conclusions, Comment: #4 of a series of 4 legacy reviews on QPC's

We demonstrate that low concentrations of a secondary magnetic phase in (Ga,Mn)As thin films can enhance the coercivity by factors up to ~100 without significantly degrading the Curie temperature or saturation magnetisation. Magnetic measurements indicate that the secondary phase consists of MnAs nanoclusters, of average size ~7nm. This approach to controlling the coercivity while maintaining high Curie temperature, may be important for realizing ferromagnetic semiconductor based devices., Comment: 8 pages,4 figures. accepted for publication in Appl. Phys. Lett

We report on a comprehensive study of the ferromagnetic moment per Mn atom in (Ga,Mn)As ferromagnetic semiconductors. Theoretical discussion is based on microscopic calculations and on an effective model of Mn local moments antiferromagnetically coupled to valence band hole spins. The validity of the effective model over the range of doping studied is assessed by comparing with microscopic tight-binding/coherent-potential approximation calculations. Using the virtual crystal k.p model for hole states, we evaluate the zero-temperature mean-field contributions to the magnetization from the hole kinetic and exchange energies, and magnetization suppression due to quantum uctuations of Mn moment orientations around their mean-field ground state values. Experimental low-temperature ferromagnetic moments per Mn are obtained by superconducting quantum interference device and x-ray magnetic circular dichroism measurements in a series of (Ga,Mn)As semiconductors with nominal Mn doping ranging from ~2% to 8%. Hall measurements in as-grown and annealed samples are used to estimate the number of uncompensated substitutional Mn moments. Based on our comparison between experiment and theory we conclude that all these Mn moments in high quality (Ga,Mn)As materials have nearly parallel ground state alignment., Comment: 11 pages, 11 figures, submitted to Phys. Rev. B

We report the results of a detailed study of the structural, magnetic and magnetotransport properties of as-grown and annealed Ga0.91Mn0.09As thin films grown on (311)A and (311)B GaAs substrates. The high Curie temperature and hole density of the (311)B material are comparable to those of GaMnAs grown on (001) GaAs under the same growth conditions, while they are much lower for the (311)A material. We find evidence that Mn incorporation is more efficient for (311)B than for (001) and significantly less efficient for (311)A which is consistent with the bonding on these surfaces. This indicates that growth on (311)B may be a route to increased Curie temperatures in GaMnAs. A biaxial magnetic anisotropy is observed for the (311) material with easy axes along the [010] and [001] out-of-plane directions. An additional uniaxial in-plane anisotropy is also observed with the easy axis along for the (311)A material, and along for the (311)B material. This new observation may be of importance for the resolution of the outstanding problem of the origin of uniaxial anisotropy in (001) GaMnAs., Comment: 16 pages, 6 figures and 1 table. accepted by Phys. Rev. B

We demonstrate that the interplay of in-plane biaxial and uniaxial anisotropy fields in (Ga,Mn)As results in a magnetization reorientation transition and an anisotropic AC susceptibility which is fully consistent with a simple single domain model. The uniaxial and biaxial anisotropy constants vary respectively as the square and fourth power of the spontaneous magnetization across the whole temperature range up to T_C. The weakening of the anisotropy at the transition may be of technological importance for applications involving thermally-assisted magnetization switching., Comment: 4 pages, 4 figures

We have performed a systematic investigation of magnetotransport of a series of as-grown and annealed Ga1-xMnxAs samples with 0.011 <= x <= 0.09. We find that the anisotropic magnetoresistance (AMR) generally decreases with increasing magnetic anisotropy, with increasing Mn concentration and on low temperature annealing. We show that the uniaxial magnetic anisotropy can be clearly observed from AMR for the samples with x >= 0.02. This becomes the dominant anisotropy at elevated temperatures, and is shown to rotate by 90o on annealing. We find that the in-plane longitudinal resistivity depends not only on the relative angle between magnetization and current direction, but also on the relative angle between magnetization and the main crystalline axes. The latter term becomes much smaller after low temperature annealing. The planar Hall effect is in good agreement with the measured AMR indicating the sample is approximately in a single domain state throughout most of the magnetisation reversal, with a two-step magnetisation jump ascribed to domain wall nucleation and propagation., Comment: 27 pages, 8 figures, accepted by Phys. Rev. B

We report on a comprehensive combined experimental and theoretical study of Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad agreement between theoretical expectations and measured data allows us to conclude that T_c in high-quality metallic samples increases linearly with the number of uncompensated local moments on Mn_Ga acceptors, with no sign of saturation. Room temperature ferromagnetism is expected for a 10% concentration of these local moments. Our magnetotransport and magnetization data are consistnent with the picture in which Mn impurities incorporated during growth at interstitial Mn_I positions act as double-donors and compensate neighboring Mn_Ga local moments because of strong near-neighbor Mn_Ga-Mn_I antiferromagnetic coupling. These defects can be efficiently removed by post-growth annealing. Our analysis suggests that there is no fundamental obstacle to substitutional Mn_Ga doping in high-quality materials beyond our current maximum level of 6.2%, although this achievement will require further advances in growth condition control. Modest charge compensation does not limit the maximum Curie temperature possible in ferromagnetic semiconductors based on (Ga,Mn)As., Comment: 13 pages, 12 figures, submitted to Phys. Rev. B

We report on measurements of the crystal structure and hole density in a series of as-grown and annealed GaMnAs samples. The measured hole densities are used to obtain the fraction of incorporated Mn atoms occupying interstitial and substitutional sites. This allows us to make a direct comparison of the measured lattice parameters with recent density functional theory (DFT) predictions. We find that the decrease in lattice constant observed on annealing is smaller than that predicted due to the out diffusion of interstitial Mn during annealing. The measured lattice parameters after annealing are still significantly larger than that of GaAs even in samples with very low compensation. This indicates that the intrinsic lattice parameter of GaMnAs is significantly larger than that of GaAs, in contradiction to the DFT prediction., Comment: To appear in Appl. Phys. Lett.,13 pages,3 figures and 1 table

We have investigated the magnetic properties of (Ga,Mn)As thin films with Mn concentration between 1 and 9%. Ferromagnetic transition temperatures TC of up to 173K are observed. The results are compared to the predictions of the Zener mean-field theory. We find no evidence of a fundamental limit to TC., Comment: presented at ICPS-27; 2 pages

Results of magnetisation measurements on p-type zincblende-(Ga,Mn)N are reported. In addition to a small high temperature ferromagnetic signal, we detect ferromagnetic correlation among the remaining Mn ions, which we assign to the onset of hole-mediated ferromagnetism in (Ga,Mn)N., Comment: 2 pages, 1 figure, proc. ICPS 27, Flagstaff '04

We show, by SQUID magnetometry, that in (Ga,Mn)As films the in-plane uniaxial magnetic easy axis is consistently associated with particular crystallographic directions and that it can be rotated from the [-110] direction to the [110] direction by low temperature annealing. We show that this behavior is hole-density-dependent and does not originate from surface anisotropy. The presence of uniaxial anisotropy as well its dependence on the hole-concentration and temperature can be explained in terms of the p-d Zener model of the ferromagnetism assuming a small trigonal distortion., Comment: 4 pages, 6 Postscript figures, uses revtex4

We report a large tunneling anisotropic magnetoresistance (TAMR) in a thin (Ga,Mn)As epilayer with lateral nanoconstrictions. The observation establishes the generic nature of this effect, which originates from the spin-orbit coupling in a ferromagnet and is not specific to a particular tunnel device design. The lateral geometry allows us to link directly normal anisotropic magnetoresistance (AMR) and TAMR. This indicates that TAMR may be observable in other materials showing a comparable AMR at room temperature, such as transition metal alloys., Comment: 4 pages, 4 figures

Cubic (zinc-blende) Ga1-xMnxN layers were grown by plasma-assisted molecular beam epitaxy on GaAs (001) substrates. Some of the structures also contained cubic AlN buffer layers. Auger Electron Spectroscopy and Secondary Ion Mass Spectroscopy studies clearly confirmed the incorporation of Mn into cubic GaN, the Mn being uniformly distributed through the layer. X-ray diffraction studies demonstrated that the Mn-doped GaN films are cubic and do not show phase separation up to a Mn concentrations of x<0.1. P-type conductivity for the cubic Ga1-xMnxN layers was observed for a wide range of the Mn doping levels. The measured hole concentration at room temperature depends non-linearly on the Mn incorporation and varies from 3x10^16 to 5x10^18 cm-3., Comment: 7 pages, 3 figures

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, and resistivity measurements during annealing, show that the observed changes are due to out-diffusion of Mn interstitials towards the surface, governed by an energy barrier of about 0.7-0.8 eV. The Mn interstitial is a double donor resulting in compensation of charge carriers and suppression of ferromagnetism. Electric fields induced by high concentrations of substitutional Mn acceptors have a significant effect on the diffusion., Comment: 5 pages, 4 figures, submitted to Physical Review Letters

We study the dc transport properties of (Ga,Mn)As diluted magnetic semiconductors with Mn concentration varying from 1.5% to 8%. Both diagonal and Hall components of the conductivity tensor are strongly sensitive to the magnetic state of these semiconductors. Transport data obtained at low temperatures are discussed theoretically within a model of band-hole quasiparticles with a finite spectral width due to elastic scattering from Mn and compensating defects. The theoretical results are in good agreement with measured anomalous Hall effect and anisotropic longitudinal magnetoresistance data. This quantitative understanding of dc magneto-transport effects in (Ga,Mn)As is unparalleled in itinerant ferromagnetic systems., Comment: 3 pages, 3 figures

The magnetoresistance in a series of Ga1-xMnxAs samples with 0.2 =< x =< 0.8 have been measured for three mutually orthogonal orientations of the external magnetic field. The anisotropy magnetoresistance decreases with increasing of the Mn content, which means that the magnetic disorder or defects scattering increases with increasing of Mn content. And also the magneto-crystalline anisotropy increases with increasing of Mn concentration., Comment: Presented at ICPS-26, July 2002

We show that by annealing Ga1-xMnxAs thin films at temperatures significantly lower than in previous studies, and monitoring the resistivity during growth, an unprecedented high Curie temperature Tc and conductivity can be obtained. Tc is unambiguously determined to be 118 K for Mn concentration x=0.05, 140 K for x=0.06, and 120 K for x=0.08. We also identify a clear correlation between Tc and the room temperature conductivity. The results indicate that Curie temperatures significantly in excess of the current values are achievable with improvements in growth and post-growth annealing conditions., Comment: 4 pages, submitted to Applied Physics Letters

The resistivity, temperature, and magnetic field dependence of the anomalous Hall effect in a series of metallic Ga1-xMnxAs thin films with 0.015=

By studying the Hall effect in a series of low resistivity Ga1-xMnxAs samples, accurate values for the hole density p, Mn concentration x, and Curie temperature Tc are obtained over the range 0.015=